(a) Field of the Invention
The present invention relates to a method for preparing polycarbonate resin, and more particularly to a method for effectively preparing polycarbonate resin having a large molecular weight under a melt polymerization condition using a catalyst system comprising phosphoranylidene ammonium salts, which is stable and maintains superior reactivity during melt polymerization and solid state polymerization.
(b) Description of the Related Art
Polycarbonate resin has superior mechanical properties, such as impact resistance, heat resistance and transparency, and therefore is widely used for a variety of machine components, optical discs, automobile components, and so forth.
Conventionally, polycarbonate was prepared by interfacial polycondensation of bisphenol, such as bisphenol A, and phosgene. It was also prepared by melt polymerization or solid state polymerization, wherein bisphenol and carbonic acid diester (e.g., diphenyl carbonate) are transesterified.
In the interfacial polycondensation, the polymerization is carried out while dissolving polymers in a solvent. As a result, when preparing aromatic polycarbonates with high degree of polymerization, viscosity of the solution increases excessively so that additional efforts and time are required for purification, neutralization, etc. of polymers.
By using the transesterification method (i.e., polymerization with transesterification of bisphenol and carbonic acid diester), polycarbonates can be prepared at lower cost than using the interfacial polycondensation, and toxic materials like phosgene or methylene chloride are not used. Therefore, this method has attracted attention for the recent years.
In the transesterification method, polycarbonates are prepared by reacting bisphenol and carbonic acid diester under a high-temperature and reduced-pressure condition in the presence of specific catalysts.
Currently, a metal compound catalyst system and a nonmetal compound catalyst system are known. For the metal compound catalyst system, U.S. Pat. No. 3,153,008 discloses an organometal compound comprising salt of alkali metal or alkali earth metal, such as hydroxide, acetate, alkoxide, carbonate, hydride and oxide, and transition metals like zinc, cadmium, titanium or lead. And, U.S. Pat. No. 4,330,664 discloses aluminum hydride or borohydride.
For the nonmetal compound catalyst system, U.S. Pat. No. 3,442,854 discloses a compound represented by the following Chemical Formula I:

In Chemical Formula I:
R1, R2, R3 and R4 are hydrocarbons; Z is nitrogen, phosphorus or arsenic; and X is tetraaryl borohydride, bromide, phenolate or diaryl phosphate.
In addition to these nonmetal compound catalyst systems, U.S. Pat. No. 5,168,112 discloses primary, secondary or tertiary amine and nitrogen-containing aromatic compound derivatives like pyridine; U.S. Pat. No. 5,418,316 discloses guanidine and its derivatives; U.S. Pat. No. 5,618,906 discloses phosphazine compounds; and U.S. Pat. No. 6,262,219 discloses nitrogen-containing ring compounds like piperidine or morpholine.
The conventional metal compound catalyst systems have problems such that reactivity of the metal compound catalyst systems decreases significantly at low concentration. And, if the concentration is too high, polycarbonates having irregular branches are generated, and thereby impairing chromaticity and lowering stability. The conventional nonmetal compound catalyst systems also have problems such that although the nonmetal compound catalyst systems show good reactivity at low temperature, a large amount of catalyst is required to have reactivity comparable to that of the metal compound catalyst systems. Also, the catalyst itself decomposes at the reaction temperature and converts to a substance with low boiling point. As a result, the high-temperature reactivity becomes deteriorated.
Accordingly, a need exists for a method for effectively preparing polycarbonate resin under a melt polymerization condition using a catalyst system, which is stable and maintains superior reactivity during melt polymerization and solid state polymerization.